CI: rename runner label ubuntu-latest → py3.12-node22

The label now explicitly declares the dependency versions it provides
rather than using the opaque ubuntu-latest alias. All runs-on fields
in ci.yml updated to match.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

infra/act-runner-config.yml

@@ -11,7 +11,7 @@ runner:
   # Maps runs-on labels to Docker images for job containers.
   # Change this (and delete /data/.runner) to switch the base image.
   labels:
-    - "ubuntu-latest:docker://git.fabledsword.com/bvandeusen/runner-base:py3.12-node22"
+    - "py3.12-node22:docker://git.fabledsword.com/bvandeusen/runner-base:py3.12-node22"
 
 cache:
   enabled: true